I. Field of the Invention
The present invention relates to radio communications. More particularly, the present invention relates to power control in a code division multiple access cellular radiotelephone system.
II. Description of the Related Art
Multiple access techniques are some of the most efficient techniques for utilizing the limited radio frequency spectrum. Examples of such techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA wireless technology, governed by Electronic Industry Association/Telecommunication Industry Association Interim Specification--95 (IS-95), employs a spread spectrum technique for the transmission of information. A spread spectrum system uses a modulation technique that spreads the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required to transmit the signal.
A form of frequency diversity is obtained by spreading the transmitted signal over a wide frequency range. Since only part of a signal is typically affected by a frequency selective fade, the remaining spectrum of the transmitted signal is unaffected. A receiver that receives the spread spectrum signal, therefore, is affected less by the fade condition than a receiver using other types of signals.
The spread spectrum technique is accomplished by modulating each base band data signal to be transmitted with a unique wide band spreading code. Using this technique, a signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz. Typical examples of spread spectrum techniques are found in M. K. Simon, Spread Spectrum Communications, Volume I, pp. 262-358.
In a CDMA-type radiotelephone system, multiple signals are transmitted simultaneously at the same frequency. A particular receiver then determines which signal is intended for that receiver by a unique spreading code in each signal. The signals at that frequency, without the particular spreading code intended for that particular receiver, appear to be noise to that receiver and are ignored.
Since multiple radiotelephones and base stations transmit on the same frequency, power control is an important component of the CDMA modulation technique. A higher power output by a radiotelephone and/or base station increases the interference experienced by the other radiotelephones and base stations in the system. In order to keep the radiotelephones and base stations from transmitting too much power, thereby lowering system capacity, some form of power control must be implemented.
The power control is performed by two power control loops: open loop and closed loop power control. The radiotelephone, in performing the open loop control, uses a received signal strength indicator, well known in the art, to estimate the signal path power loss from the base station to the radiotelephone. The radiotelephone can then compensate the power loss incurred over the forward channel by transmitting at a higher power level over the reverse channel.
Since the loss on the forward channel may not be the same as the loss on the reverse channel, the open loop power control is just an initial estimate that must be corrected. The correction is the closed loop power control. This is accomplished by the base station transmitting a power control bit to the radiotelephone instructing the radiotelephone to adjust its transmit power in response to the channel conditions.
If the power control algorithm is perfect, the radiotelephone transmitter is able to perfectly match the losses and distortions induced on its signal by the channel. However, since there is a delay between the base station determining the power control bit to be transmitted and the radiotelephone actually receiving the power control bit, the channel conditions may have changed.
If the channel encountered by the radiotelephone signal changes a great deal over the delay period, the radiotelephone may not be able to adjust its transmit power to match the current channel conditions. This may cause serious problems with the system capacity. There is a resulting unforeseen need for a faster power control process that takes into account the time varying channel conditions.